To improve the responsive and the obstacle climbing capability of tracked robot, a new underactuated tracked robot is proposed,which is based on differential gear. Crawler, rocker leg, wheel and other mobile modes are analyzed when robot is in soft, rugged and flat terrain. Four modes of overcoming vertical wall for the robot are described. The relationship between driving torque and vertical wall height, and the relationship between driving torque and size of the robot are calculated by building dynamics model. The kinematics and dynamics simulations in ADAMS are carried out, and torque curve of the obstacle crossing process is obtained. The principle prototype is established to perform the obstacle climbing experiments. Theoretical analysis and experimental results show that the robot can change the mode of movement and increase the responsive, and many ways to overcome obstacles can enhance obstacle climbing capability of the robot.